INTRODUCTION
In the past, parasitic infections were regarded as a major public health problem in Korea. Over the past three decades, the prevalence of such infections has been decreasing rapidly, accompanied by an increase in the GNP, improvements in sanitation and hygiene, changes in agricultural management, and a nationwide control plan (
Cho, 1994;
Rim, 1997). Infection by soil-transmitted helminths, such as
Ascaris and
Trichuris, decreased dramatically: the egg-positive rates for these parasites were 13.0-23.4% in 1981 and 0.04-0.06% in 1997 (
MHW and KAH, 1997). However, the pattern of prevalence of snail-transmitted trematode infections seems to be quite different from that of soil-transmitted helminth infections. The nationwide egg-positive rates for
Clonorchis sinensis and
Metagonimus sp. were 2.6% and 1.2% in 1981, and 1.4% and 0.3% in 1997, respectively (
MHW and KAH, 1997). These data show that clonorchiasis and metagonimiasis are still common parasitic diseases in Korea.
C. sinensis, the liver fluke, is a well-known parasite that is of major public health importance in Korea and several other Asian countries. Infection with
C. sinensis occurs by eating uncooked freshwater fish that harbors the metacercariae. In Korea, human infection has been shown to have a high prevalence along several major Korean rivers (
Seo et al., 1981;
Kim et al., 1994;
Rim, 1997). The Geum-gang (River) basin is a well-known endemic area, where the prevalence of clonorchiasis over the last two decades has been in the range of 30.8 to 50.7% (
Chang, 1979;
Seo et al., 1981;
Kim et al., 1994). Metagonimiasis is a fish-borne trematodiasis that is also of medical importance in Korea; it is endemic along the southern and eastern coasts (
Seo et al., 1981;
Kim et al., 1987;
Chai et al., 1993 and
2000). In addition to these areas, the Geum-gang (River) basin, western coastal areas, and the Namhan-gang (River) basin have also been reported to be endemic areas (
Kim 1980;
Chai et al., 1993;
Kim et al., 1994).
Even today, snail-transmitted trematodes infections are the most significant parasitic diseases in Korea, especially in rural and riverside areas, and the Geum-gang basin is an endemic area for clonorchiasis and metagonimiasis. Although there have been a few surveys of the prevalence of snail-transmitted trematode infection in this area in the past (
Chang, 1979;
Kim, 1980;
Seo et al., 1981;
Kim et al., 1987;
Kim et al., 1994), there is no up-to-date survey reports. Therefore, we examined the endemicity and intensity of infection by
C. sinensis and
Metagonimus sp. in inhabitants of Okcheon-gun (County) living near the Geum-gang (River), and also obtained the follow up results after treatment of
C. sinensis-positive cases with praziquantel.
DISCUSSION
Okcheon-gun lies in the Geum-gang (River) basin in Chungcheongbuk-do, Korea. Among the 1,081 inhabitants surveyed in this area, we found overall egg-positive rates of 9.3% for C. sinensis and 5.5% for Metagonimus sp.. The mean EPG counts for egg-positive cases were 918±1,463 and 711±947, respectively. The egg-positive rates for both snail-transmitted trematodes were significantly higher in both males or inhabitants of the riverside area than in other groups. These data show that the egg-positive rates for both parasites were significantly lower than those reported earlier in similar regions. However, this area still has a high prevalence of infection with these parasites.
The prevalence of soil-transmitted helminth infections in Korea has sharply decreased over the last three decades. The overall helminthes egg-positive rate was 84.3% in 1971, 41.1% in 1981, 3.8% in 1992, and 2.4% in 1997 (
MHK and KAH, 1997). We also confirmed very low egg-positive rates of soil-transmitted helminthes; among the 1,081 subjects surveyed, only five cases of soil-transmitted helminthes were found. The egg-positive rates for
C. sinensis decreased steadily from 4.6% in 1971, 2.6% in 1981, and 2.2% in 1992, to 1.4% in 1997 (
MHK and KAH, 1997). A nationwide survey of clonorchiasis showed that endemic areas were scattered throughout the country along several major rivers, including the Geum-gang (River), and the most intensive endemic regions were mainly found along the Nakdong-gang (
Seo et al., 1981;
Rim, 1997). Clonorchiasis is a common infectious disease in Korea. For example, the yearly egg-positive rate for
C. sinensis among outpatients nationally ranged from 2.3 to 3.9% between 1984 and 1992 (
Lee et al., 1994), indicating the infection of this trematode infection in the general population.
The Geum-gang basin is one of the endemic regions for snail-transmitted trematode infections in Korea. The egg-positive rate for
C. sinensis in Okcheon-gun was 50.7% in 1979 (
Chang, 1979), 40.4% in 1994 (
Kim et al., 1994), and 9.3% in the present study, confirming that the local prevalence of clonorchiasis has significantly decreased over the past 6 years. Whereas Kim et al. (
1994) previously surveyed the riverside within 1 km of the main channel of the river, the present study examined both the riverside and the inland areas within 5 km from the main channel. Generally, the prevalence of
C. sinensis is higher in males and in inhabitants living in rural areas, and increases with age (
MHW and KAH, 1997;
Rim, 1997). We also found that egg-positive rates were higher among inhabitants of the riverside area than in those of the inland area, and also higher in males than in females. Although the fifties were the highest egg-positive rates for
C. sinensis in both riverside and inland areas, there were no significant differences of egg-positive rates among age groups, except the thirties. However, the egg-positive rates of the fifties or forties were significantly higher rates than that of the other age groups in the riverside areas, except age group of 0-9 years. The EPG counts were also much lower than those previously reported. In the Geum-gang basin, the mean EPG value was 5,760 (range, 100-26,600) in 1981 (
Seo et al., 1981), whereas it was 918±1,463 in this study. The low EPG counts in this area may be due to the government-supported control program, which includes praziquantel treatment and health education. However, these EPG counts may not be enough to develop apparent clinical features in humans, thus they may have repeated to eat the raw fish. The EPG for
Clonorchis in most cases with clinical symptoms and abnormal liver function tests is more than 10,000 (
Rim et al., 1981).
Three species of
Metagonimus are known to occur in Korea;
M. yokogawai (
Chai and Lee, 1990),
M. miyatai (
Kim et al., 1987;
Chai et al., 1993), and
M. takahashii (
Chai and Lee, 1990).
M. yokogawai is widely distributed in large and small rivers of the southern and eastern coasts of Korea (
Seo et al., 1981;
Chai et al., 2000). According to a previous survey along the Geum-gang basin, human infections with
Metagonimus sp. different from
M. yokogawai were first noticed by Kim (
1980), and Kim et al. (
1987) later reported it as
Metagonimus Miyata type. In this study, we did not classify the species of
Metagonimus. In the Geum-gang basin in Okcheon-gun, the egg-positive rate for
Metagonimus sp. was 25.9 % in 1979 (
Chang 1979) and 9.8% in 1994 (
Kim et al., 1994). In this study, the egg-positive rate and EPG count for
Metagonimus sp. were 5.5% and 711±947, which are lower than those observed in the previous reports (
Chang, 1979;
Kim, 1980;
Seo et al., 1981;
Kim et al., 1994).
Praziquantel is a very well tolerated drug; occasional side effects consist of mild and transient headache and dizziness. In this study, C. sinensis-positive cases were given 3×25 mg/kg in one day at the Okcheon-gun Public Health Center. Five of the 85 specimens collected after praziquantel treatment were positive for C. sinensis eggs. Treatment failure in these 5 cases was thought to be due to either an insufficient treatment dosage of the drug or reinfection after treatment. The most practical method of preventing human reinfection is to avoid eating raw or undercooked freshwater fishes. All five treatment failure cases had eaten raw freshwater fishes after praziquantel treatment.
Human infection with C. sinensis and Metagonimus sp. is usually acquired by eating uncooked fishes containing infectious metacercariae. The intensity of human infection is dependent upon the eating habits of the population. In rural and riverside areas of Korea, eating raw fish is a deeply rooted traditional custom. The results described in this study indicated that the overall prevalence for C. sinensis and Metagonimus infections in Okcheon-gun was significantly lower than that in the previous reports. However, this area still has high prevalence of clonorchiasis and metagonimiasis. Therefore, practical prevention or health education programs as well as mass chemotherapy are needed to reduce the prevalence of snail-transmitted trematodiasis in endemic areas.